The present disclosure relates in general to solid oxide fuel cell power plants, and more particularly, to a system and method for satisfying temperature and pressure demands of a solid oxide fuel cell power plant in a manner that decreases the weight and increases the operating efficiency of the plant.
Solid oxide fuel cell (“SOFC”) power plants are emerging as an attractive technology for mobile applications. Fuel cells generate electrical energy and heat by electrochemically combining a gaseous fuel, such as hydrogen, and an oxidant, such as oxygen, across an ion-conducting electrolyte. In solid oxide fuel cells, the electrolyte is an oxygen ion conductive ceramic membrane sandwiched between an oxygen electrode (cathode) and a fuel electrode (anode). Oxygen reacts with electrons at the cathode to form oxygen ions, which are conducted through the ceramic membrane to the anode, where the oxygen ions combine with hydrogen and carbon monoxide to form water and carbon dioxide.
A typical SOFC power plant may be grouped into subsystems including but not limited to power generation, fuel processing, and air preparation subsystems. In general, the power generation subsystem comprises at least one fuel cell including an anode flow field for receiving a flow of fuel gas, a cathode flow field for receiving a flow of oxygen gas, and a ceramic electrolyte membrane separating the anode from the cathode flow fields. The fuel processing subsystem includes a reformer for generating hydrogen-rich gas fuel, frequently referred to as reformate or syngas, from a hydrocarbon fuel source for feeding to the anode channel of the fuel cell. Hydrogen gas fuel may be generated from a variety of common hydrocarbon fuel sources using methods including steam reformation, autothermal reformation, and catalytic partial oxidation. The air preparation subsystem supplies oxygen to the cathode flow field of the fuel cell, normally by drawing atmospheric air through a blower to pressurize the air to an adequate level for delivery to the cathode flow field, in addition to providing air for reformation, if needed.
Each subsystem typically has strict temperature, pressure, weight, efficiency, and other requirements. For example, the electrochemical reactions in a solid oxide fuel cell are typically performed at temperatures of between around 550-1000° C., thus necessitating that the temperature of air entering the cathode flow field be between around 450-750° C. Furthermore, to ensure adequate delivery of oxygen to the cathode electrode, the air provided to the cathode flow field must be at a pressure sufficient to overcome the pressure drop across the flow field, for example. Similarly, air delivered to fuel reformers utilizing an atomizer and a catalytic reactor must be at a pressure sufficient to overcome the even higher pressure drop associated with these components. Furthermore, the air must be heated to a temperature sufficient for ensuring the adequate mixing of fuel and air for catalytic reformation to generate a hydrogen-rich reformate gas stream for feeding to the anode flow field of the fuel cell, for example, above about 350° C. However, prior to entering the anode flow field, the reformate stream may need to be cooled such that the effective operating temperature of the fuel cell is not exceeded. In addition to the thermal management and pressure requirements listed above, SOFC systems for mobile applications must be compact and lightweight.